Thin-film magnetic recording media formed on a rigid disc substrate are widely used in read/write memory devices in computers. The demand for data storage capacity in such devices has increased dramatically in recent years and disc drives using inductive head technology have a storage capacity, i.e., areal density, on the order of 400 Mb/in.sup.2, and those using magnetoresistive heads are over 600 Mb/in.sup.2 (Russak, M. R., et al., Proc. Electrochem. Soc., 95(18):143-156 (1996)).
The storage capacity of magnetic media can be improved in several ways, including increasing media coercivity, reducing the magnetic remanence-thickness product, reducing media noise and decreasing the flying height.
A high coercivity is one factor important in achieving a high recording density in a magnetic disc. Coercivity, defined as the magnetic flux required to reduce the remanence magnetic flux to zero, i.e., the field required to erase a stored bit of information. A higher coercivity in a medium allows adjacent recorded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density.
Magnetic remanence, M.sub.r, provides a measure of the signal amplitude which can be read from an isolated pulse stored in a magnetic medium. The greater the remanence, or moment, the greater the signal amplitude which can be detected in a reading operation.
Signal-to-noise ratio (SNR) is the ratio of signal amplitude, or peak-to-peak amplitude of a single pulse, as a function of recording frequency, to recording noise at that frequency. A high SNR, due to sinal amplitude and/or low noise, is necessary for high density recording.
Widely used commercial thin-film media are prepared by sputtering a magnetic thin film on a substrate, such as a textured, plated aluminum substrate. The disc is typically prepared by sputtering a chromium underlayer onto the substrate surface, then sputtering the magnetic thin film recording layer over the underlayer.
One approach to improving disc storage density by increasing the coercivity has been to vary the composition and method of preparing the underlayer. For example, an increase in the thickness of the chromium underlayer results in an increased coercivity. (Fisher, R. D. et al., IEEE Trans. on Magn., 22:352 (1986); Johnson, K. E., J. Appl. Phys, 67:4686 (1990)).
Another approach to improving coercivity, and hence storage capacity, is to employ certain magnetic alloys which result in improved coercivity, for example, by including platinum in a CoCr-based magnetic recording film.
Another approach to increasing media coercivity is to apply a negative voltage bias to the substrate during sputtering. Typically, an improvement in coercivity is observed when the bias is applied to the substrate during sputtering of the magnetic recording layer, with no improvement, or a reduction in coercivity, achieved with the bias is applied during sputtering of the chromium underlayer (Lal, B. B. and Russak, M. A., J. Appl. Phys., 81(8):3934 (1997)). A voltage bias during sputtering of both the chromium-based underlayer and the magnetic layer can also improve coercivity (Lal, B. B. and Russak, M. A., J. Appl. Phys., 81(8):3934 (1997); U.S. Pat. No. 5,069,983). Others have reported modest increases in coercivity when a bias is applied to the substrate during sputtering of the chromium underlayer (U.S. Pat. No. 5,147,734).